The earliest reference to isothermal flow was found
in Shapiro's Book.
The model suggests that the choking occurs at
and it appears that Shapiro was the first one to realize this
difference compared to the other models.
In reality, the flow is choked somewhere between
to
one for cases that are between Fanno (adiabatic) and isothermal flow.
This fact was evident in industrial applications
where the expectation of the choking is at Mach one, but
can be explained by choking at a lower Mach number.
No experimental evidence, known by the undersigned, was ever
produced to verify this finding.